In the start up of paper-making machines and in the event of web ruptures therein, the paper web can be removed at several different points along the paper-making machine. If the sheet being formed in the forming section cannot immediately be transferred to the press section of the machine, the separation of the sheet from the forming wire must as a rule take place after the couch or head roll of the machine with the sheet being knocked down into a couch pit provided for this purpose. In general, water jets are used to knock down the sheet. Fresh water is usually used for this which also requires a corresponding amount of discharge of fibre-containing waste water from the circulation system of the paper-making machine. The jets that knock down and separate the sheet from the wire normally require high pressures in the range of 10 to 15 kp/cm.sup.2 and the required water quantity is about 200 to 500 liters/minute for each meter of paper machine width. Considering environment preservation aspects it is therefore desirable that new alternatives to the depression jets be found. In certain paper-making machines attempts have been made to solve the problem by using internal cleansing of the waste water to be used for the water jets on the web. In some cases purified waste water is used also for the depression jets. However, this is not an altogether satisfactory solution as complete cleansing cannot be achieved and because there are risks of clogging of the spray nozzles which as a rule consist of circular openings having a diameter of 1 to 2 millimeters, and also risks for fibre formation on the inner face of the wire and on the leading rolls in the return part of the wire.
It has also been found that water jets used to depress the fibre web decrease in efficiency as the speed of the wire in the paper-making machine is increased. Metal wires have to a large extent been replaced on modern machines by synthetic forming fabrics. When plastic forming fabrics are used it is considerably more difficult to separate the web from forming fabric as compared with the use of metal wires, particularly at high machine speeds. Additionaly, when the web is separated from the wire it is often dragged along with the wire beyond the wire pit or towards the leading rolls in the wire return part. Attempts have been made to solve this problem by increasing the pressure and the quantity of water in the knock down spray pipes with little success.
In the removal of a paper web from the forming wire a distinction is made between the two following, separate operational steps, viz.:
1. Removal of a paper web and transfer thereof onto a felt. This step may be performed in machines similar to the one described in the U.S. Pat. No. 2,990,013.
2. Knock-off of the paper web into a couch pit provided for this purpose. Such knock-off takes place in connection with web breaks and when the paper web is allowed to accompany the wire somewhat further but is to be removed before the wire passes the first return roll. Applicants' invention is directed to this latter operation.
In addition, various spray apparatuses and methods are used in the industry to clean the wire and for edge-cutting purposes. Modern development of paper machines and forming wires has made it more difficult to knock off the paper web. The increases in machine speed and use of synthetic forming wires have complicated things considerably. To counter-act the weaker dimensional stability of the synthetic resin (plastics) wires the so-called double-layer synthetic wires are frequently used. These however present the disadvantage of making knock-off of the paper web more difficult.
It is common to provide one or several shower tubes of high-pressure type for knock-off. These showers include nozzles which are positioned at a fixed distance from one another. In old machines and machines of reduced dimensions the knock-off shower operates continuously and under normal operational conditions serves as a cleaning shower. Large, up-to-date machines usually comprise sensor means of some kind to detect web breaks, which sensors either start the function of the showers or increase the shower pressure from low-pressure spraying to high-pressure spraying. The structure in accordance with the U.S. Pat. No. 3,218,227 is in principle of an ordinary basic design equipped with a special means to rapidly start up the showers when wire breaks occur.
As mentioned earlier, the U.S. Pat. No. 2,990,013 describes a system for removal of the paper web from the wire and transfer thereof onto a felt. The system is based on a two-channel system comprising a suction slot the purpuse of which is to form a film of water between the web and the wire, and a slot emitting a jet of compressed air to lift the sheet from the wire. For the purpose of removing the paper web and transferring it to a felt the air-jet slot is the one most commonly used. It is true that an air-jet spray may detach the web from the wire but the web is moved into renewed contact with the web by the air currents caused by the forward movement of the wire.